This invention relates to a power conversion system comprising a noise reduction circuit adapted to flow a noise compensation current in order to cancel the leakage current when driving an electric motor at variable speed.
Electric motors have a wide variety of applications including elevators, rolling mills and automobiles. They normally comprise a power converter to make the electric motor adapted to supply power at a desired rate. Typical power converters comprise an inverter formed by arranging a plurality of switching devices such as GTOs (gate turn-off thyristors) and IGBTs (insulated gate bipolar transistors) both in series and in parallel. The arrangement using an inverter will be described hereinafter as a typical arrangement for realizing power converters.
In recent years, electric motor drive systems using an inverter of the above identified type are drawing attention because of the problem of ground leakage current (to be referred to simple as leakage current hereinafter) that arises when each switching element of the system is used for high speed switching operations.
FIG. 1 is a schematic circuit diagram of a power conversion system and its peripheral arrangement illustrating this problem. In the drive system of this electric motor, a 3-phase AC voltage is applied to full-wave rectifier 2 from an AC power source.
The full-wave rectifier 2 comprises diodes D1 through D6 that are connected to show 3-phase bridge connection. The 3-phase AC voltage from the AC power source is transformed into a DC voltage, which is then supplied to inverter 3 through between positive side input line P and negative side input line N. Note that the full-wave rectifier 2 and the inverter 3 constitute a power converter.
The inverter 3 comprises switching elements Q1 through Q6 connected for 3-phase bridge connection and applies a pulse-shaped (rectangular wave) voltage with a limited width to each of the winding terminals of the electric motor 4 for three phases under the control of a gate drive circuit (not shown) that operates for PWM (pulse width modulation) control. Thus, the electric motor 4 is driven by the pulse-shaped voltage.
However, the electric motor 4 has a floating capacitance C that appears between itself and the ground. Therefore, as each of the switching elements Q1 through Q6 is turned on/off and a pulse-shaped voltage is applied to the electric motor 4, a pulse-like voltage will be applied between the related terminal of the electric motor 4 and the ground.
Then, a leakage current I1 that is a noise current flows to the ground through the floating capacitance C between each of the windings of the electric motor and the frame ground as a function of the rate-of-change of the voltage dv/dt.
The leakage current I1 flows through each of the grounding lines between the electric motor 4 and the grounding terminal of the AC power source 1 into or out of the latter depending on the polarity. The leakage current I1 can give rise to operation errors of the leakage breaker of the circuit and electric shocks to the operator.
In an attempt for avoiding problems due to such a leakage current I1, the use of a noise reduction circuit as shown in FIG. 2 has been proposed for power converters.
The noise reduction circuit comprises a leakage current detector 5 for detecting the leakage current, if any, flowing from the supply line between the AC power source 1 and the full-wave rectifier 2 and a noise reduction circuit 6 adapted to flow a noise compensation current by making the stretch between positive side input line P and the ground or between the ground and the negative side input line N electrically conductive depending on the detected leakage current.
The noise reduction circuit 6 comprises an amplifier 7, an npn-type transistor Tr1, a pnp-type transistor Tr2 and a coupling capacitor C1. The transistors Tr1, Tr2 are required to show a high withstand voltage and operate at a high frequency to produce a high current amplifying effect.
The leakage current detector 5 is typically a zero-phase current transformer CT having an annular core made of ferrite and adapted to equivalently detect the leakage current I1 flowing to the full-wave rectifier 2 on the basis of the difference in the power source current and send a detection signal to the amplifier 7.
Of the transistors Tr1, Tr2, the npn-type transistor Tr1 has its collector connected to the positive side input line P and its emitter connected to the emitter of the pnp-type transistor Tr2 and also to one of the opposite ends of the coupling capacitor C1. The pnp-type transistor Tr2 has its collector connected to the negative side input line N. The other end of the coupling capacitor C1 is connected to the ground.
Thus, upon receiving an output signal from the amplifier 7 at the base, one of the transistors Tr1, Tr2 turns on the other, while the latter turns off the former so that either the positive side input line P or the negative side input line N will be grounded by way of the coupling capacitor C1.
For example, when the leakage current I1 flowing from the electric motor 4 to the grounding line of the AC power source 1, the noise reduction circuit turns on only the pnp-type transistor Tr2.
Then, the noise compensation current i flows into the closed circuit consisting of the diode D4, D5 or D6 of the full-wave rectifier 2 by way of the negative side input line N, the pnp-type transistor Tr2 and the coupling capacitor C1.
Therefore, the leakage current I1 flowing into the grounding terminal of the AC power source 1 is cancelled by the noise compensation current i.
On the other hand, when the leakage current I1 flowing from the grounding line of the AC power source 1 to the electric motor 4, the noise reduction circuit turns on only the npn-type transistor Tr1.
Then, the noise compensation current i flows from the diode D1, D2 or D3 of the full-wave rectifier 2 to the grounding line by way of the positive side input line P, the npn-type transistor Tr1 and the coupling capacitor C1.
Therefore, the leakage current I1 flowing from the grounding line of the AC power source 1 to the electric motor 4 is cancelled by the noise compensation current i.
However, in the power converter having a configuration as described above, the positive side input line P of the full-wave rectifier 2 shows the ground potential when, for instance, the diodes D3 and D4 of the full-wave rectifier 2 are held in an electrically conductive state in the full-wave rectifier 2.
Then, if the npn-type transistor Tr1 is turned on, no noise compensation current i flows from the noise reduction circuit and noise becomes uncontrollable because the positive side input line P and the ground show no potential difference.
If, on the other hand, the devices D6 and D1 of the full-wave rectifier 2 are held in an electrically conductive state, the negative side input line N of the full-wave rectifier 2 shows the ground potential. Then, if the pnp-type transistor Tr2 is turned on, no noise compensation current i flows from the noise reduction circuit and noise becomes uncontrollable because the negative side input line N and the ground show no potential difference.
This problem arises also in the arrangement of FIG. 5 where a total of n sets A1 through An, each comprising a power converter of a full-wave rectifier 2 and a inverter 3 and an electric motor 4, are connected in series. In FIG. 5, only the set A1 is illustrated in detail because all the sets A1 through An are identical. The power conversion system of FIG. 5 is provided with noise reduction circuits as shown in FIG. 6 arranged for the respective power converters. However, the arrangement of FIG. 6 is not free from the problem of uncontrollability.
Additionally, the power conversion system of FIG. 6 has as many noise reduction circuits as the number of power converters to make it less adapted to down-sizing.